1. Field of the Invention
The invention relates to a process for modifying finely divided particulate solids by gas phase silylation.
2. Description of the Related Art
Surface modified finely divided particulate solids such as silicas and specifically pyrogenic silicas are used inter alia for controlling the flow properties of adhesive, sealant and coating materials, for improving the mechanical properties of elastomers, and for controlling the charge and flow properties of powders such as toners or powder finishes.
The surface modification of the finely divided particulate solids is preferably a chemical, irreversible modification in that the modifying agent is attached to the particle surface via a chemical bond. Typical modifying agents to enter a chemical reaction with the particle surface are organosilanes or organosiloxanes. These bind with the filler surface to form stable solid-silicon-oxygen bonds.
Processes for surface modification of finely divided particulate solids with organosilanes or organosiloxanes are known, for example from DE 10150274 or EP 0686676.
The common feature of the processes described therein is a first step of intensively mixing the unmodified finely divided particulate solids with the organosilane and the organosiloxane, respectively, these being added to the solid either as a vapor or as a very finely divided aerosol. The solid and the organosilane/organosiloxane are then made to react thermally, followed by a purifying operation in which any adherent elimination products or excess modifying reagents are removed from the particle surface. Preferably, the offgases generated in the individual processing steps are at least partly recycled into the mixer in order to limit losses of organosilane/organosiloxane. It transpired, however, that there are technical reasons why complete recycling of the offgases is not possible and at least a portion of the offgases has to be flued from the plant in order to prevent the accumulation of elimination products and supply fresh N2 for inertization.
The offgas to be flued first has to be freed of entrained solids. This is accomplished using known methods of separating solids from gas streams such as, for example, filters with or without cyclones.
The organosilanes/organosiloxanes in the offgas stream are withdrawn from the reaction and thus lead to a significant reduction in the yield of product based on the amount of organosilane/organosiloxane used. The organosilicon compounds in the offgas stream further lead to fouling and hence blinding of the filter fabric, so pressurization occurs upstream of filters and the system has to be shut down to change the filter. This leads to a significant increase in costs through the increased consumption of filter materials and manufacturing outage. Reducing the plant throughput does extend the filter on-stream time, but only at the expense of the manufacturing output. The problem of filter fabric blinding due to fouling is aggravated when using sparingly volatile organosilicon compounds such as, for example, organosiloxanes that are not vaporizable without decomposition and thus are in the state of a finely divided aerosol when they become mixed with the finely divided particulate solids.
EP 1184425 A describes a process for surface modification of silicas with organohalosilanes which comprises a step of both mixing and reacting the silica with vaporous, i.e. volatile, organohalosilane in a fluidization vessel. A sub-stream of the silica to be modified is metered into the offgas line of the fluidization vessel, then separated from the offgas and finally returned into the fluidization vessel for reaction, while the temperature in the fluidization vessel is in the range from 400 to 600° C. This process is disadvantageous because the sub-stream of the silica separated from the offgas is metered directly into the hot nitrogen-fluidized reaction zone and thereby gives rise to inhomogeneously modified products, i.e., products having a widely varying degree of silylation. The process described also involves a complex, difficult-to-control operation featuring additional vessels and valves to return the sub-stream into the reaction after it has been separated.